1. Field of the Invention
The present invention relates to a wafer processing method for processing a wafer having a plurality of bumps formed on its front surface.
2. Description of the Related Art
In the field of semiconductor manufacturing, wafers tend to become larger year after year and the wafers are made thinner for increasing packing density. In order to make a semiconductor wafer thinner, a backgrinding process for grinding a back surface of the wafer is performed. In the backgrinding process, a surface protection film is applied to a front surface of the wafer for protecting chips formed on the front surface of the wafer.
After the backgrinding process is finished, a dicing film application device applies a dicing film to the back surface of the wafer, so that the wafer is integrated with a mount frame. Then, the surface protection film that has been applied to the front surface of the wafer is removed, and then, the wafer is diced into chips. Each chip formed by the dicing process is picked up and mounted on a lead frame.
FIG. 8 is a cross-sectional view of a wafer before the backgrinding process in the conventional art. As illustrated in FIG. 8, bumps B as electrical contacts are formed on each chip C on the front surface of the wafer. Because of chips C and bumps B formed thereon, front surface 21 of wafer 20 is not flat. As can be seen from FIG. 8, when surface protection film 110 is applied to front surface 21 of wafer 20, a central region of wafer 20 is elevated from its peripheral region. In other words, a step is formed on the front surface protection film 110.
As illustrated in FIG. 9, wafer 20 is turned upside down and mounted on a table 51 for grinding the back surface 22 of wafer 20. Table 51 is provided with a suction section 52 that is slightly smaller than the outer shape of wafer 20. Because surface protection film 110 is elevated in the central region, it is not in contact with suction section 52 in the peripheral region. Therefore, as illustrated in the figure, in the peripheral region, an annular clearance is formed between the surface protection film 110 and suction section 52.
In this state, when back surface 22 of wafer 20 is ground, stress may be concentrated on the central region of wafer 20 and, as a result, bumps B located in the central region and/or wafer 20 itself may be damaged. Further, because the peripheral region of wafer 20 is not sucked by suction section 52, the edge of wafer 20 may be broken during the backgrinding process. Still further, during the backgrinding process, shavings may be sucked through the annular clearance by suction section 52 and, as a result, suction section 52 may be clogged.
In order to solve these problems, Japanese Unexamined Patent Publication No. 2005-109433 discloses a technique for covering the front surface of the wafer by a cup-shaped protection member that is provided with a peripheral adhesive part adhering to the peripheral region of the wafer. When such protection member is used, the stress during the backgrinding process is distributed over the entire wafer, and therefore the above-mentioned problems can be avoided.
However, the protection member disclosed in Japanese Unexamined Patent Publication No. 2005-109433, which is a special-purpose product conforming only to the bumps of a particular size, is expensive and therefore the cost of the resultant semiconductor chip is increased. Considering the fact that this protection member is used only during the backgrinding process and eventually discarded, it is desirable to reduce the cost of the protection member as much as possible.
Further, because the protection member is the special-purpose product, even when the height of the bumps is changed only slightly, the protection member cannot protect the bumps properly. In this case, if the protection member is used as it is without modification, the problems such as the damage to the wafer and/or bumps, the breakage of the edge of the wafer, and the clogging of the suction section may occur again.
The present invention has been made in view of such circumstances and it is an object of the present invention to provide an inexpensive wafer processing method that can easily accommodate change of size of bumps without creating clearance between a wafer and a table.